Monolithic photodetector

ABSTRACT

A photodetector including a photodiode formed in a semiconductor substrate and a waveguide element formed of a block of a high-index material extending above the photodiode in a thick layer of a dielectric superposed to the substrate, the thick layer being at least as a majority formed of silicon oxide and the block being formed of a polymer of the general formula R 1 R 2 R 3 SiOSiR 1 R 2 R 3  where R 1 , R 2 , and R 3  are any carbonaceous or metal substituents and where one of R 1 , R 2 , or R 3  is a carbonaceous substituent having at least four carbon atoms and/or at least one oxygen atom.

PRIORITY CLAIM

This application claims priority from French patent application No.06/50536, filed Feb. 14, 2006, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to the field ofimagers made in monolithic form. More specifically, embodiments of thepresent invention relate to the structure of photodetectors used in suchimagers.

2. Discussion of the Related Art

Fixed or mobile image acquisition devices are increasingly used in manyfields. They must be inserted into smaller and smaller spaces. Forexample, image acquisition devices are inserted into portable phones.According to another example, in the medical field, it is desirable tohave image acquisition devices of small dimensions able to be arrangedon endoscopes.

It has thus been attempted to make, in monolithic form, imageacquisition devices of small dimensions with an image quality at leastcomparable to that of known optical devices.

A monolithic image acquisition device includes photodetectors arrangedat the intersection of lines and of columns of an array.

A cross-sectional view of FIG. 1 illustrates four photodetectors of aline or column of an image acquisition device. A photodetector comprisesa photodiode D formed in a semiconductor substrate 1. The surface areataken up by photodiode D typically has a side or a diameter rangingbetween 0.2 and 1.5 μm. Substantially above the interval separating twoneighboring photodiodes D, metal interconnects are formed in adielectric 5. The metal interconnects are formed of several levels ofmetallization 3 and of vias. The thickness of dielectric 5 is generallygreater than the total height of metallizations 3 and depends ontechnological constraints linked to standard technological processesand/or to the circuits formed around the image acquisition area, such ascircuits for reading and processing the acquired images. The thicknessof dielectric 5 typically ranges between 1.5 and 6.5 μm.

A succession of filters F of transparent resin colored in red R, greenV, or blue B is formed in dielectric 5. Filters F follow one another sothat, on a same line, two different colors alternate and that, on twosuccessive lines, one color is common. For example, a first linecomprises filters according to the sequence B-V-B-V and the next linecomprises filters according to the illustrated sequence R-V-R-V. FiltersF follow one another in a continuous fashion and the interface betweentwo neighboring filters F is substantially above the middle of theinterval separating two underlying photodiodes D. Thus, eachphotodetector comprises a filter F associated with a photodiode D. Thefilter F of each photodetector is topped with a respective converginglens L, also made of transparent resin.

It has been provided to form in each photodetector, between a filter Fand its corresponding photodiode D, a waveguide G. Waveguide G is formedof a waveguide block 7 surrounded with dielectric 5. Block 7 is formedof a material with a refraction index n₇ greater than that, n₅, ofdielectric 5 (n₇>n₅). Block 7 is of straight or slightly conicalcylindrical shape. Block 7 is placed above lens L to receive the photonsinjected by lens L towards photodiode D. The high portion of block 7 isseparated from filter F by a thickness of dielectric 5 negligible as tothe induced light losses, typically on the order of a few nanometers.Similarly, the bottom of block 7 is placed above photodiode D and isseparated therefrom by a thickness of dielectric 5 on the order of a fewnanometers. The presence of waveguide G enables decreasing lightintensity losses and avoiding wrong detections linked to a dispersion ofthe photons and/or to their refraction against the metallizations 3which appear across the relatively significant thickness of dielectric5.

According to one example of the waveguide G, the guide is formed of acone of a silicon, oxygen, carbon, and nitrogen compound called siliconoxynitride, which exhibits an index n on the order of from 1.6 to 2.3according to its stoichiometry formed in a thick silicon oxide layer(SiO₂) of index n=1.43. Another example of the waveguide G uses tantalumoxide, which has an index n on the order of 2, as a high-index material.

However, the use of such materials may raise practical problems. Inparticular, block 7 of FIG. 1 of the present application is formed byfilling a deep and narrow opening in the dielectric 5. “Deep and narrow”means in the present description an opening having a ratio between thedepth (substantially equal to the thickness of dielectric 5) and theaverage diameter (substantially equal to that of photodiode D) greaterthan 2. The filling of such an opening, which is performed by chemicalvapor deposition (CVD), must be homogeneous. However, on filling of anarrow and deep opening with compounds of silicon oxy-carbo-nitride typeor with tantalum oxide, bubbles or cavities form. Such bubbles formtraps for the received light Further, when block 7 is formed of asilicon oxy-carbo-nitride or of tantalum oxide, problems of mechanicalhold with peripheral silicon oxide 5 can be observed. Moreover, some ofthe silicon oxy-carbo-nitrides, as well as the tantalum oxide,deteriorate during the thermal cycles implemented in the rest of theprocess, especially the encapsulation and packaging anneals performed attemperatures from 300 to 400° C.

In another approach, a waveguide element is formed of alumina of index1.63 or of silicon nitride (n=1.83) formed in a thick silicon oxidelayer n=1.43. According to a variation of this approach, the high-indexblock 7 is silicon oxide n=1.43, formed in a thick dielectric layer 5made of a material with a lower index such as an oxysilane of index1.39.

The use of alumina or silicon nitride in a thick silicon oxide layer mayexhibit disadvantages similar to those described previously for tantalumoxide or silicon oxy-carbo-nitrides.

The use of silicon oxide in oxysilane may also raise problems. Inparticular, this results in a significant modification of the materialsused in the optical area with respect to the material present in theneighboring non-optical areas in which it is desirable to keep siliconoxide as an interlevel dielectric 5. This complicates and increasesmanufacturing costs. Further, the use in a thick layer of oxysilane witha refraction index lower than that of silicon oxide raises problems ofmechanical hold, of ability to be locally etched, especially to formmetallization levels 3 and the vias, as well as problems of resistanceto thermal stress, especially on forming of the metal levels.

SUMMARY OF THE INVENTION

One embodiment of the present invention is to provide a waveguide blockwhich overcomes at least some of the disadvantages of known structures.

Another embodiment of the present invention is to provide such a blockwhich is compatible with the use of silicon oxide as a thick peripheraldielectric.

Another embodiment of the present invention is to provide such awaveguide block which is compatible with the thermal cycles implementedafter its forming and to provide such a block which is easy tomanufacture in a narrow and deep opening.

According to one embodiment of the present invention, a photodetectorincludes a photodiode formed in a semiconductor substrate and awaveguide element formed of a block of a high-index material extendingvertically above the photodiode in a thick layer of a dielectricsuperposed to the substrate, the thick layer being formed at least as amajority, of silicon oxide and the block being formed of a polymer ofthe general formula R₁R₂R₃SiOSiR₁R₂R₃ where R₁, R₂, and R₃ are anycarbonaceous or metal substituents and where one of R₁, R₂, or R₃ is acarbonaceous substituent having at least four carbon atoms and/or atleast one oxygen atom.

The block may exhibit a ratio between its depth and its average diametergreater than 2. A colored filter may be placed above the block and aconverging lens may also be placed above the block. The lens may belaterally offset with respect to the block. The polymer according to anembodiment of the present application may be an SOG-type glasscomprising tantalum, titanium, and/or zirconium inclusions. An imageacquisition device may include a plurality of such photodetectors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the present invention will be discussed indetail in the following non-limiting description of specific embodimentsin connection with FIG. 1, previously described, which is a partialcross-sectional view of a known image acquisition device.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments will be readily apparent to those skilled in the art, andthe generic principles herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentinvention. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein. As usual in therepresentation of semiconductor components, FIG. 1 is not drawn toscale.

Referring to FIG. 1, an embodiment of the present invention uses a block7 formed of a polymer of the general formula R₁R₂R₃SiOSiR₁R₂R₃ where R₁,R₂, and R₃ are any carbonaceous or metal substituents such as tantalum,titanium, or zirconium compounds and where one of R₁, R₂, or R₃ is acarbonaceous substituent having at least four carbon atoms and/or atleast one oxygen atom and the polymer having a refraction index greaterthan the refraction index of silicon oxide which is equal to 1.43, thisblock being buried in the silicon oxide.

Such polymers exhibit many advantages when used for forming thewaveguide blocks 7. First, as indicated previously, the refraction indexof such polymer is greater than that of the silicon oxide dielectric 5in which the block is formed. Interlevel dielectric 5 then is the samein the optical areas including the photodiodes D as in the neighboringnon-optical areas. This simplifies the fabrication process.

Further, as they are spun on and not deposited by chemical vapordeposition (CVD) like conventionally-provided materials, theabove-described polymers can be deposited in the deep and narrowopenings having a ratio between depth and average diameter of greaterthan 2.

Moreover, during the deposition, such polymers polymerize in a chain,which reliably and reproducibly results in the forming of a homogeneousblock 7, that is, with no cavities or bubbles.

Furthermore, the compatibility between such a polymer and a peripheralsilicon oxide (dielectric 5) is good. There is no phenomenon ofmechanical separation between these polymers and silicon oxide, nor isthere any cross contamination. There are no longer wetting defectslikely to result in the forming of cavities.

Additionally, the above-defined polymers present a high range barrierpower against the diffusion of the metal elements constituting thecolored resins of the filters F of FIG. 1. This barrier is higher thanthe barrier of the known materials as silicon oxynitride, tantalumoxide, alumina, silicon oxide silicon nitride, methylsiloxane, silicatesor phosphosilicates. Such a barrier property is highly advantageous. Itavoids deteriorating the filters during the fabrication process due tothe diffusion of the metal elements of the colored resins in the waveguide. It increases the performances and the lifetime of an imager usingsuch a waveguide. Especially, it deeply reduces the loss of performancesdue to the above-described diffusion during the life of the imager.

Moreover, such polymers exhibit a good resistance to temperaturesgreater than 400° C. Thus, these polymers are thermally compatible withsemiconductor circuit manufacturing processes.

The block 7 may be formed, for example, of a quasi-inorganicsiloxane-based insulator of SOG (spun on glass) type, including or notmetallic tantalum, titanium, and/or zirconium inclusions and having arefraction index ranging between 1.56 and 1.6, such as manufactured byTokyo Ohka Kogyo. Another possible polymer is a siloxane material,having a refraction index ranging between 1.64 and 1.7 such as thatmanufactured by Silecs Inc.

The present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, only those elements of the monolithicstructure of a photodetector which are necessary to the understanding ofthe present invention have been described. It will be within theabilities of those skilled in the art to complete the structure to forman operative device. Further, it should be understood by those skilledin the art that the present invention applies to photodetectors ofdifferent structures. For example, a photodetector may comprise nofilter F or lens L.

Further, it should be understood by those skilled in the art thatdielectric 5 has been considered as homogeneous as a non-limitingexample only and for clarity. In practice, dielectric 5 may be made ofsilicon oxide only across the greatest part of its thickness, that is,it may have a multiple-layer structure. In particular, dielectric 5 istypically formed of an alternation of thick silicon oxide sub-layersseparated by thin sub-layers of an etch stop insulator such as siliconnitride. The presence of such thin sub-layers does not affect theoperation of the photodetectors.

Moreover, embodiments of the present invention have been described inthe case of a block 7 exhibiting a ratio between its depth and itsaverage diameter greater than 2. However, it should be understood bythose skilled in the art that such blocks may exhibit a ratio betweentheir depth and their average diameter which is less than 2.

Further, it should be understood by those skilled in the art thatwaveguides according to embodiments of the present invention have beendescribed as applied to a device as illustrated in FIG. 1 as anon-limiting example only. Embodiments of the present invention alsoapply to image acquisition devices exhibiting a different generalstructure. Thus, for example, colored filter F and lens L may altogetherbe radially offset with respect to underlying block 7 to correct anangle of view which varies from the center of the sensor to the edge ofthe sensor. In this case, it should be noted that it will be preferableto form between each block 7 and filter F a significant thickness of thedielectric, for example, on the order of from 200 to 1,500 nm.

Generally, it will be within the abilities of those skilled in the artto select an appropriate polymer from among the polymers available forsale with a refraction index greater than that of the dielectric 5 thatis used.

Image acquisition devices including arrays of photodetectors accordingto embodiments of the present invention may be any type of electronicdevice containing an image acquisition device, such as cellular phones,digital cameras, video cameras, personal digital assistants (PDAs), andso on.

Alterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and the scopeof the present invention. Accordingly, the foregoing description is byway of example only and is not intended to be limiting. The presentinvention is limited only as defined in the following claims and theequivalents thereto.

1. A photodetector comprising a photodiode formed in a semiconductorsubstrate and a waveguide element formed of a block of a high-indexmaterial extending vertically above the photodiode in a thick layer of adielectric superposed to the substrate, the thick layer being at leastas a majority formed of silicon oxide and the block being formed of apolymer having the general formula R₁R₂R₃SiOSiR₁R₂R₃ where R₁, R₂, andR₃ are any carbonaceous or metal substituents and where one of R₁, R₂,or R₃ is a carbonaceous substituent having at least four carbon atomsand/or at least one oxygen atom.
 2. The photodetector of claim 1,wherein said block exhibits a ratio between its depth and its averagediameter of greater than
 2. 3. The photodetector of claim 1, wherein acolored filter is placed above the block.
 4. The photodetector of claim1, wherein a converging lens is straight above the block.
 5. Thephotodetector of claim 1, wherein a converging lens is placed above theblock and is laterally offset with respect to said block.
 6. Thephotodetector of claim 1, wherein the polymer is an SOG-type glasscomprising tantalum, titanium, and/or zirconium inclusions.
 7. An imageacquisition device, comprising a plurality of photodetectors of claim 1.